The classical Greek elements earth, air, fire, and water were long used to explain the way things moved. Fire lacked weight and would lift objects to the heavens. Earth was heavy, and would pull them to the ground. Every object was thought to be some mix of these four basic elements, which in turn governed how it moved.

Silly as this sounds, it had the potential for being a viable theory if the Greeks had made an effort to better define what the four elements were. Instead, their logic was circular. A rock falls to the ground because it contains so much earth. And what is earth? It’s that which makes a rock fall to the ground.

This kind of thinking rankled natural philosophers of the seventeenth century. Addressing a related theory, Descartes’ wrote, “[The theory was] introduced by philosophers … to account for the … action of natural things, … But no natural action at all can be explained by [them], since [as] their defenders admit … they are occult.”

Descartes’ and his contemporaries were busy with new theories that tossed out the occult properties of objects and replaced them with a universe of objects bumping into one another. Heat could be explained by the rapid movement of small objects called “atoms”; sight was light striking the eyes. This new way of looking at the world came to be known as mechanical philosophy.

Champions of mechanical philosophy saw their activities as more than simply putting forward a new theory. They saw themselves as slayers of ignorance. And in actuality they were. But dogmatism in any form can lead to trouble.

Which is what happened when Sir Isaac Newton proposed his theory of gravity. Gravity, it seemed, wasn’t the result of matter colliding with matter. It was a property of matter, leaving it open to accusations of being occult.

The answer? Describe gravity in terms of objects bumping into other objects so that Newton’s gravity and mechanical philosophy could peacefully coexist. This required filling space with an as yet unseen aether – a physical substance that would make gravity work. No such aether was ever found, and aether theories were eventually abandoned, though their demise took centuries.